1. Field of The Invention
The present invention relates to tungsten based alloys, and more particularly to a method for heat-treatment of tungsten based alloys, capable of improving impact toughness, namely, impact energy while keeping tensile strength and elongation.
2. Description of the Prior Art
Tungsten based heavy alloys contain tungsten of at least 90 weight %. They also contain nickel, iron and/or copper.
Referring to FIG. 1, there is illustrated a typical microstructure of the tungsten based alloys. As shown in FIG. 1, the microstructure comprises spherical tungsten grains (BCC) and a matrix phase (FCC) which consists of nickel, iron and tungsten or of nickel, copper and tungsten. The tungsten based alloys are usually manufactured by a liquid phase sintering process which is a kind of powder metallurgy. This liquid phase sintering process is illustrated in FIG. 2.
Tungsten based heavy alloys exhibit a high density of 16 to 19.1 g/cm.sup.3, a superior tensile strength of 700 to 950 MPa, a high elongation of 5 to 25% and a superior formability or machinability, depending on their alloy compositions such as tungsten, nickel, iron and/or copper contents. Thus, the tungsten based heavy alloys are widely used in fields requiring both a small volume and a heavy weight.
For example, the tungsten based heavy alloys are widely used for rotors, balance weights for aircrafts and shield materials against radioactive rays in commercial industry fields and core materials for kinetic energy penetrators in military industry fields. Recently, the speed of rotors and aircrafts is on an increasing trend. Such a trend involves a requirement for increasing a stability of structural elements against fracture. Where the tungsten heavy alloys are used as the materials for penetrators, it is believed that the penetration depth increases with increasing impact toughness.
As mentioned above, the tungsten heavy alloy is a kind of composite material comprising hard tungsten grains and a ductile matrix phase and having two kinds of characteristic interfaces, namely, tungsten-matrix and tungsten-tungsten interfaces. It is known that the bonding strength of the tungsten-matrix interface is higher than that of the tungsten-tungsten interface. Accordingly, the impact toughness of tungsten heavy alloys is considerably dependant on the relative fraction of tungsten-tungsten and tungsten-matrix interfaces.
On the other hand, it is also known that the bonding strength of the tungsten-matrix interfaces is greatly decreased due to a segregation of impurities. For maximizing the impact toughness of tungsten heavy alloys, accordingly, it is required to minimize both the relative fraction of the tungsten-tungsten interfaces and the segregation of impurities in the tungsten-matrix interfaces.
Now, the variation in bonding strength caused by the segregation of impurities in the tungsten-matrix interfaces will be described.
The decrease of bonding strength at the tungsten-matrix interfaces is caused by the fact that impurities such as phosphorous, sulphur and carbon contained in the raw materials and hydrogen are segregated in the tungsten-matrix interfaces, due to the difference in solubility. Thus, for enhancing the impact toughness of tungsten heavy alloys, it is required to remove the hydrogen and to prevent the segregation of impurities at tungsten-matrix interfaces by using a heat-treatment after sintering.
Referring to FIG. 3 there is illustrated a conventional heat-treatment proposes for removing the hydrogen and preventing the segregation of impurities. This heat-treatment comprises the steps of maintaining a sintered tungsten heavy alloy at a temperature of 1,000.degree. to 1,200.degree. C. in an atmosphere of an inert gas such as nitrogen or argon or in a vacuum and then water quenching.
The purpose of the maintaining at such a high temperature is to remove the remaining hydrogen and to diffuse out the segregation of impurities. On the other hand, the water quenching makes it possible to prevent the re-segregation of impurities. Hence, the heat-treatment greatly contributes to an increase in impact toughness of the tungsten heavy alloy.
Although the impact toughness is increased by solving the problems of the brittleness caused by the hydrogen and the segregation of impurities at tungsten-matrix interfaces, the conventional heat-treatment method has a limitation on the increase in impact toughness, in that the relative fraction of tungsten-tungsten interfaces which are the most brittle interfaces of tungsten heavy alloys can not be controlled.
Accordingly, it is required to develop a method for changing brittle tungsten-tungsten interfaces to strong tungsten-matrix interfaces so as to obtain an improvement in impact toughness.